After beating her sister Venus in the third round of the U.S. Open last week, Williams took time to praise Kaepernick and former 49ers safety Eric Reid for their courage.

"I think every athlete, every human, and definitely every African American should be completely grateful and honored how Colin and Eric are doing so much more for the greater good, so to say," Williams said. "They really use their platform in ways that is really unfathomable.

"I feel like they obviously have great respect from a lot of their peers, especially other athletes, people that really are looking for social change."

Kaepernick, who has been out of the NFL since he took a knee during the national anthem in 2016, was part of a new Nike ad campaign released Monday.